[ss4vegeta1]

Hi I am new to this forum I have been studying for a test today and I am trying to understand kinetics. My biggest problem is understanding why when doing kinetics the order is a first, second or third order problem. I know it has to do with the exponents but how can you tell? Someone please answer I have a test this week. After using 2 rates and 2 experiments, I calculated 1 for one and 2 for the other does that make [A]= first order and [ B]= second order in respect to [A]? Also what is there is a one to one ratio like Ex2HI(g) ----> H2(g) + I2(g) my textbook says its second order but it also says there is no way we could ever have predicted this without expermental data and one should never assume the coefficents are the same cause it is a trap. I am tottally clueless. Thanks so much everyone.

[Astrokel]

Order of reactants can only be determined experimentally. From the example u provided,
 2HI(g) ----> H2(g) + I2(g) , a stoichiometric coefficient of 2 for HI(g) does not mean the order for [HI] is 2 in the rate law. Remember the order is determined via experiments.

[ss4vegeta1]

Remember the order is determined via experiments? Do you mean that chart showing experiments 1-3? Can you elaborate more please?
Or can someone add to this?

[Astrokel]

What charts?

You can only determine the orders of reactants via experiments.

Lets say u want to find out the orders of reactants of 2A + B ---> C with a catalyst D

You set up experiments by varying the concentrations of A, B and D and calculate the initial rates.

  [A]     [ B]     [D]    Intial rate
   1        1        2       1.00 x10^-5    ---- (1)
   1        2        2       2.00 x 10^-5   ---- (2)
   2        2        2       2.00 x 10^-5   ---- (3)
   2        2        1       0.50 x 10^-5   ---- (4)

By comparing data (1) and (2), you notice when [ B] increased by 2 times, initial rate also increased by 2 times. Therefore [ B] has an order of 1.

If you were to compare (2) to (3), when [A] is increased by 2 times, intial rate stays the same. Thereby [A] has an order of 0 despite its stoichiometric coefficient is 2 in the reaction.

Comparing (3) and (4), you notice when [D] is decreased by 2 times, intial rate decrease by 4 times. Therefore, [D] has an order of 2.

Hence, overall rate law is r = k[ B][D]^2

Hence, you see that the stoichiometric coefficients can't tell you the orders of reactants, its only possible through experimental datas.

Kelvin